minor #47672 Leverage First-class callable syntax (tigitz)

This PR was merged into the 6.2 branch.

Discussion
----------

Leverage First-class callable syntax

| Q             | A
| ------------- | ---
| Branch?       | 6.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | -
| License       | MIT
| Doc PR        | -

### Rationale
https://wiki.php.net/rfc/first_class_callable_syntax

Mainly:
>  The advantage is that the new syntax is accessible to static analysis, and respects the scope at the point where the callable is created.

I'd argue that it also improves readability and IDE color syntax also helps:

![image](https://user-images.githubusercontent.com/1524501/191912084-7ee933c5-dda1-4176-86f1-cd6511c58aa4.png)

I've manually reviewed each changes and discarded some of them where `[Foo::class, 'method']` was intended to be tested with this specific syntax

Commits
-------

26d9ce9520 Leverage First-class callable syntax

Author: nicolas-grekas

Tests/Constraints/UniqueValidatorTest.php

@@ -162,6 +162,7 @@ public function getCallback(): array
             }],
             'callable with string notation' => ['Symfony\Component\Validator\Tests\Constraints\CallableClass::execute'],
             'callable with static notation' => [[CallableClass::class, 'execute']],
+            'callable with first-class callable notation' => [CallableClass::execute(...)],
             'callable with object' => [[new CallableClass(), 'execute']],
         ];
     }